The present invention relates to a control system for compressed gas refrigeration dryers.
Control systems for compressed gas refrigeration dryers are, for example, described in European patents EP-1089803 and EP-1103296.
A compressed gas refrigeration dryer is a refrigeration machine which is normally used for extracting humidity from a certain flow of compressed air or any other compressed gas; in this respect, even though reference is made hereunder to compressed air, it is evident that all disclosures are equally valid for any other compressed gas.
Humidity present in compressed air is the main cause of corrosion and premature breakage of piping and the malfunctioning or complete uselessness of machines using compressed gas, and it must therefore be eliminated before supplying compressed air to the above equipment.
In the traditional functioning of a compressed air refrigeration dryer, the compressed air which enters the dryer is cooled to condense the humidity (water vapour), which is entrained therewith, subsequently separating and discharging the condensed water vapour; the compressed air leaving the dryer consequently has a much lower dew point than that at the inlet.
All refrigeration compressed gas dryers are generally characterized by a cooling load (a combination of flow-rate, temperature, pressure and relative humidity of the gas) varying from 0 to 100%, and the presence of a suitable control system which adapts the refrigerating capacity produced to the cooling load, at the same time preventing the temperature of the compressed gas from dropping below the value of 0xc2x0 C., to avoid the formation of ice, and attempting to obtain a constant dew point over a period of time.
In particular, there are currently four different types of compressed gas refrigeration dryers.
The first type of dryer, commonly called xe2x80x9chot gas dryerxe2x80x9d, normally comprises a refrigerating compressor, which operates uninterruptedly, and a xe2x80x9chot gasxe2x80x9d by-pass valve which adapts the refrigerating capacity produced to the cooling load and maintains the temperature of the refrigerant, which evaporates, to values higher than 0xc2x0 C. The advantages of this system however, relating to the on-variability of the dew point and relatively low costs, contrast with the considerable waste of energy, which is a serious disadvantage, above all nowadays when energy saving is particularly required.
Another type of gas dryer, called xe2x80x9cthermal mass dryerxe2x80x9d, uses an evaporator immersed in a thermal mass (liquid or solid), whose temperature is controlled by a thermostat which activates the compressor when the temperature of the evaporator exceeds a pre-established value; this type of dryer basically functions in the same way as a household refrigerator and maintains the temperature of the thermal mass at values higher than 0xc2x0 C.
In this case, however, a desirable energy saving is accompanied by difficulties in obtaining a constant dew point and extremely high production costs.
In a further variation of the dryer, the rotation rate of the compressor is controlled by an electronic device (inverter) which adapts the refrigerating capacity produced to the cooling load, maintaining constant temperature and pressure values of the refrigerant, which evaporates at values higher than 0xc2x0 C.; below a certain rotation rate (within a range of values equal to about 50-80% of the nominal value), this dryer functions in the same way as a dryer of the xe2x80x9chot gasxe2x80x9d type.
In this case, a constant dew point is obtained and there is a reasonable energy saving, but still higher than that obtained with a thermal mass dryer.
In the last type of compressed gas dryer (xe2x80x9cdigital scrollxe2x80x9d), the refrigerating capacity produced by a xe2x80x9cscrollxe2x80x9d-type compressor is adapted to the cooling load by a particular regulating system, which puts the compressor xe2x80x9cunder vacuumxe2x80x9d for a period of time which is proportional to the cooling load to be transferred to the evaporator.
Also in this case a constant dew point is obtained together with a good energy saving, with respect to the dryers previously described, even though the production and operating costs remain quite high.
An objective of the present invention is therefore to indicate a control system for compressed gas refrigeration dryers, which allows the refrigerating capacity or yield, produced by the refrigeration cycle, to be adapted to the cooling load (combination of flow-rate, temperature, pressure and relative humidity of the gas) to be transferred to the evaporator, which can vary from 0 to 100%, and to more or less proportionally reduce the electric power absorbed to obtain values similar to or better than those of thermal mass dryers.
Another objective of the present invention is to create a control system for compressed gas refrigeration dryers, which avoids the formation of ice in the evaporator, maintaining the gas temperatures lower than 0xc2x0 C., in order to prevent possible breakage of the evaporator and/or obstructions in the gas passage section.
Yet another objective of the present invention is to indicate a control system for compressed gas refrigeration dryers, which allows a substantially constant dew point of the gas leaving the evaporator to be obtained (as close as possible to 0xc2x0 C.) within wide load variation ranges, at the same time producing a better energy saving with respect to the known solutions.
These and other objectives are achieved by a control system for compressed gas refrigeration dryers according to claim 1, to which reference is made for the sake of brevity.